There are encoders serving as devices that detect the rotation amount or rotary position of a rotating object, such as the rotation axis of a rotary motor. Such an encoder includes a scale that has a predetermined optical pattern formed thereon and includes a magnet for generating a predetermined magnetic field, an optical sensor that detects the optical pattern, and a magnetic sensor that detects a magnetic pattern. Known examples of a scale for such a encoder include a scale in which a magnet is disposed on a surface on which an optical pattern is formed (e.g., see Patent Literature 1) and a scale in which a magnet is disposed on a surface opposite to a surface on which an optical pattern is formed (e.g., see Patent Literature 2). The optical sensor and magnetic sensor are mounted on a common circuit board and disposed so as to be opposed to the substrate surface of the scale on which the optical pattern is formed.
This encoder detects the rotation amount or rotary position of the rotation axis by rotating the scale integrally with the rotation axis, reading the optical pattern using the optical sensor, and detecting changes in the magnetic field using the magnetic sensor.